en06000621_ansi.vsd
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ANSI06000621 V1 EN
Figure 96: Distance IED on adjacent
power lines are influenced by
the negative impedance
Normally the first zone of this protection must be delayed until the gap flashing has taken
place. If the delay is not acceptable, some directional comparison must also be added to
the protection of all adjacent power lines. As stated above, a good protection system must
be able to operate correctly both before and after gap flashing occurs. Distance protection
can be used, but careful studies must be made for each individual case. The rationale
described applies to both conventional spark gap and MOV protected capacitors.
Special attention should be paid to selection of distance protection on shorter adjacent
power lines in cases of series capacitors located at the line end. In such case the reactance
of a short adjacent line may be lower than the capacitor reactance and voltage inversion
phenomenon may occur also on remote end of adjacent lines. Distance protection of such
line must have built-in functionality which applies normally to protection of series
compensated lines.
It usually takes a bit of a time before the spark gap flashes, and sometimes the fault current
will be of such a magnitude that there will not be any flashover and the negative
impedance will be sustained. If equation
72
EQUATION1898 V1 EN (Equation 72)
in figure 97, the fault current will have the same direction as when the capacitor is
bypassed. So, the directional measurement is correct but the impedance measured is
negative and if the characteristic crosses the origin shown in figure
97 the IED cannot
operate. However, if there is a memory circuit designed so it covers the negative
impedance, a three phase fault can be successfully cleared by the distance protection. As
soon as the spark gap has flashed the situation for protection will be as for an ordinary
fault. However, a good protection system should be able to operate correctly before and
after gap flashing occurs.
Section 8 1MRK 506 369-UUS -
Impedance protection
198 Line distance protection REL670 2.2 ANSI
Application manual
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